1. Field of the Invention
The present invention relates to a method for equalizing temperature differences in molten glass upstream from a tap-off point at which the glass is tapped into a mould in a forming machine. Moreover, the invention relates to an equalizer, i.e. a channel in which temperature differences in the glass melt are equalized, said channel having its outlet at to the tap-off point.
2. Description of Related Art
In the production of glass products such as glass bottles and containers of different types, it is of prime importance for the glass melt to have a predetermined and uniform weight and is viscosity. If the weight and viscosity are not uniform, the yield drops sharply. This is because the molds are not filled sufficiently, and as a result the glass bottles do not have sufficient wall thickness and do not have the necessary strength.
The glass is melted in a glass furnace from which it is transported in the liquid state via a number of transport channels. In these transport channels, attempts are made to maintain a predetermined glass temperature while keeping the temperature in the glass melt as uniform as possible. Each transport channel leads into a so-called equalizer which comprises a relatively short channel with a typical length of a few meters or so. The purpose of the equalizer is to keep the glass melt at a very uniform temperature.
The viscosity of the glass is highly dependent on temperature. Consequently, local temperature differences in the transport channel, and particularly in the equalizer, will heavily influence production yield calculated as the weight of the produced products vis-a-vis the weight of the glass melt that leaves the tap-off point.
In conventional transport channels and qualizers, a mixture of heating zones and cooling zones is used along the transport channel and equalizer. The intention is to first cool the glass to an appropriate casting temperature and then equalize the temperature in the glass melt at the predetermined casting temperature so that it is uniform throughout a cross section of the glass melt taken at right angles to the longitudinal direction of the equalizer. The cooling zones usually comprise zones where no heating takes place. Instead, to the glass melt is permitted to cool down naturally. The heating zones usually incorporate heating with a gas burner, and here the flue gas sweeps along the exposed top surface of the glass melt, but resistor heating elements are also placed along the channel walls. In addition, molybdenum electrodes are inserted in the channels in such a way that the electrodes are surrounded by the glass melt, and electric current flows through the glass melt between the two electrodes.
In conventional facilities, the glass melt temperature is measured at a number of discrete points in the glass melt using thermocouples. These measured values are used to control the heating equipment. Experience has shown that it does not suffice to measure the temperature at a number of discrete points and, on this basis, control the heating equipment due to the fact that there are still local temperature gradients at the outer boundary surfaces of the glass melt.
The present invention solves this problem and comprises a method and equipment that provide a significantly more uniform temperature in the glass melt than provided by conventional technology, and this in turn provides a substantial increase in yield.
The present invention thus relates to a method for equalizing temperature differences in molten glass in at least one temperature equalization zone in the form of a channel intended to transport a glass melt, said zone being located upstream from a tap-off point at which the glass is tapped into a mold in a forming machine or the like. Resistor heating elements are provided in the temperature equalization zone walls, bottom and roof, and the temperatures of the surfaces of the respective walls, bottom and roof contacted by the resistor heating elements are caused to be measured. The resistor heating elements are controlled by an electric controller so that the temperatures of said surfaces are equal to or largely equal to a predetermined tapping temperature of the glass melt.
Moreover, the invention also relates to temperature equalizer apparatus.